This invention relates generally to a lightweight metal piston for an internal combustion engine and in particular to a piston having a reinforced upper surface for improved erosion resistance, crack resistance and creep resistance.
It has been common for several years to manufacture pistons from lightweight metals such aluminum, magnesium, titanium and alloys of aluminum, magnesium and titanium. The use of lightweight metals in a piston reduces the mass and inertia of the piston, improving the fuel economy of the engine. However, many lightweight metals are not able to withstand the conditions encountered in operation. For example, in diesel engines it is not uncommon to include a precombustion chamber with each cylinder chamber from which a flame propagates into the cylinder chamber and impinges upon the surface of the piston crown. Without some form of reinforcement, the flame will erode the crown surface of a lightweight metal piston.
To prevent erosion of the crown surface, a ferrous heat plug may be used to protect the crown surface. The heat plug consists of an insert resembling a short, four-stroke engine valve in appearance. The heat plug has a circular top surface which covers a portion of the crown upper surface to protect the crow surface from the flame. The heat plug includes a stem which penetrates the crown of the piston. The heat plug is retained in place by threading a retaining nut onto the stem from the underside of the crown. The heat plug dissipates the heat generated at the crown surface during flame propagation from the precombustion chamber and also provides an erosion resistant surface against the jetting flame front. To function effectively, the heat plug must be tightly affixed to the piston in order to provide excellent heat transfer and effect a tight seal of the combustion gases.
However, at some point in the life of a heat plug piston, generally between 1,000 and 2,000 hours of operation, cracks form in the crown which radiate outward from the heat plug. The cracking is caused by thermal cycling which occurs as the engine responds to its duty cycle. As the engine operating hours extend, the cracks grow in length, width, and depth. Eventually, the proliferation of cracking can result in gas penetration and finally torching of the piston crown.
In addition, the thermal cycling can cause material creep or relaxation. This in turn can result in loosening of the heat plug. Once loosening occurs, movement of the heat plug in the piston crown may cause the retaining nut to release and the plug to float in the combustion chamber causing catastrophic engine failure.
Accordingly, it is an object of this invention to overcome the disadvantages of a heat plug piston by providing a reinforcement of the piston crown surrounding the heat plug to resist cracking and material relaxation.
The invention reinforces the aluminum piston crown with reinforcing fibers which impart material characteristics to the crown to inhibit the formation of thermal cracks, improve the creep resistance of the crown and improve the erosion resistance of the crown surface. The invention utilizes reinforcing fibers in the form of a cylindrical preform typically prepared by a vacuum forming process. The fiber preform is incorporated into the piston metal matrix alloy through a pressure casting process commonly used to manufacture pistons.
Research has shown that the physical and mechanical properties of a monolithic alloy can be significantly influenced by the selective addition of the reinforcing fibers. For example, it has been found that physical properties such as thermal expansion and thermal conductivity and mechanical properties such as strength, hardness, fatigue, and wear can be modified by relatively small additions of reinforcing fibers.
The problems of thermal cracking and creeping in conventional heat plug pistons is primarily related to the large disparity in elevated temperature strength and coefficient of thermal expansion between the aluminum piston and the steel heat plug. The thermal expansion coefficient of aluminum can be brought closer to that of steel and the elevated temperature yield strength of the aluminum nearly doubled by the appropriate addition of reinforcing fibers. The placement of the reinforcing fibers in the piston crown, around the ferrous heat plug, favorably changes the characteristics of the piston such that formation of radial thermal fatigue cracks is substantially retarded and the growth of the cracks is subdued. The potential creep in the crown is greatly reduced so there is less tendency for the heat plug to loosen during service.
It is an advantage of the invention that the selective reinforcement of the piston crown with fibers allows the properties of the piston alloy to be adjusted to more closely match the properties of the ferrous heat plug. The congruence in the performance between the two metals extends the life and durability of the piston of the present invention over the prior art.
The present invention may incorporate several design combinations in order to achieve the objectives of extended piston life. The geometry of the reinforced area may be varied in terms of the diameter and height or thickness of the fiber preform. The composition and other characteristics of the reinforcing fibers such as diameter, length, surface coating, etc. may be selected from any of wide range of reinforcing fibers. These include alumino-silicate, alumina, silicon carbide, silicon nitride, boron, boron carbide, and graphite. The amount and volume of the fibers may also be varied with respect to the volume of the metal alloy. The fibers may be aligned in a variety of orientations relative to the piston body. The lightweight piston metal will most commonly be aluminum and its alloys, but could also be magnesium or titanium and alloys of magnesium and titanium.
The preferred method for incorporating the reinforcing fibers into the piston metal is a squeeze casting process. However, it is possible to produce a reinforced piston using other casting techniques such as die casting or centrifugal casting.
It has also been found that by reinforcing a portion of the piston crown with ceramic fibers, that the reinforced portion provides sufficient erosion resistance to the crown surface where it is impinged with the flame front that a ferrous heat plug may no longer be required. By eliminating the ferrous heat plug, several deficiencies in prior art pistons are avoided such as the disparity in the thermal expansion and creep resistance of the two dissimilar metals.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.